1. Field of Invention
The present invention relates to a liquid crystal display device and an electronic apparatus, and more particularly relates to a reflective liquid crystal display device having an illumination device (a front light) and a directional reflector.
2. Description of Related Art
Reflective liquid crystal display devices have low power consumption since they do not have light sources, such as back lights, and conventionally, have frequently been used for, for example, accessory display portions for various portable electronic apparatuses and devices. The reflective liquid crystal display devices, however, utilize external light, such as natural light or illumination light to perform display, thus having a problem in that visibility of the display is reduced at a location where the amount of external light is limited. Thus, a liquid crystal display device having an illumination device (front light) at the front surface of a reflective liquid crystal cell has been proposed. The front light serves to introduce light from a light source, which is arranged adjacent to one end surface of a light guide plate provided at the front surface of the liquid crystal cell, into the light guide plate. The light introduced therein is then radiated from a plate surface of the light guide plate toward the liquid crystal cell. The liquid crystal display device having the front light allows the liquid crystal cell to be viewed through the transparent light guide plate at a bright location. Thus, at a bright location, the liquid crystal display device can be used as a typical reflective liquid crystal display device, and at a dark location, the front light is turned on to illuminate the liquid crystal cell, thereby allowing the display to be viewed.
FIG. 10 shows an example of a conventional front light. A front light 100 of this example includes a light source 101, a light guide plate 102 for transmitting light from the light source 101 in the rightward direction of FIG. 10, and a reflector 103 arranged so as to surround the light source 101. A plurality of projections 105 are formed on the upper surface of the light guide plate 102 at regular intervals, each projection 105 having a triangular cross-section and having a gentle-slope portion 104a with a gentle oblique angle and a steep-slope portion 104b with oblique angle steeper than the gentle-slope portion 104a. The projections 105 are formed in a striped pattern so as to extend in a direction perpendicular to the surface of the paper of FIG. 10, and serve to change the reflection direction of light propagating within the light guide plate 102 and to emit the light from a plate surface of the light guide plate 102 to the outside thereof. That is, in the front light 100, when light emitted from the light source 101 is introduced into the light guide plate 102 from a side surface thereof, the light propagates while repeating total reflection within the light guide plate 102. Meanwhile, light that was incident on the gentle-slope portion 104a of each projection 105 and that was totally reflected is emitted from the lower surface of the light guide plate 102 as an illumination light.
Meanwhile, in a reflective liquid crystal display device, a technology for providing a bright display within a certain degree of a viewing angle by forming a large number of projections or depressions on a surface of a reflective film to thereby scatter reflected light has been conventionally adopted. However, from the viewpoint of effectively utilizing reflected light in an environment where reflected light is limited and presenting a bright display to a user""s eyes, in lieu of a conventional technology in which projections or depressions each having a symmetric cross-section are formed to thereby scatter light isotropically, a technology for a so-called directional reflector has been proposed. In the technology, for example, projections each having an asymmetric cross-section, i.e., projections each having a saw-toothed cross-section and having a gentle-slope with a gentle oblique angle and a steep-slope with a steep oblique angle are formed, to impart anisotropy (hereinafter referred to as xe2x80x9cdirectivityxe2x80x9d) to light scattering, thereby orienting reflected light to the user""s eyes.
FIG. 11 is a view showing the effect of a directional reflector. For example, as shown in the figure, in a reflector 200 on which a large number of projections 201 each having a triangular cross-section are formed, the surfaces of the projections 201 serve as reflective surfaces. Designing the cross-sectional shape of each projection to have an asymmetric saw-tooth rather than to have an isosceles triangle can make the area of a gently-inclined reflective surface 202a larger enough than the area of a steeply-inclined reflective surface 202b. With an liquid crystal display device incorporating the reflector 200, in a typical operating environment, external light, such as, sunlight or illumination light, is incident on the screen from the upper direction (the direction of arrow A1 in FIG. 11) of the screen, and a user commonly views the screen from a generally frontward direction (the direction of arrow A2 in FIG. 11) of the screen. Thus, the external light from the upper direction of the screen is reflected on the gentle-slope surfaces of the reflective layer, thus increasing the amount of reflected light especially in the frontward direction of the panel. As a result, the user can view a bright image.
The conventional reflective liquid crystal display device described above has the front light and the directional reflector and is thus superior in that, in theory, it can provide a bright image regardless of an operating environment. However, in practice, when a user views the screen, there are some problems in that, for example, uneven brightness is generated depending on a location or moirxc3x3fringes are viewed. Thus, the conventional liquid crystal display device cannot necessarily be said to have good display quality.
The present invention has been made to overcome the foregoing problems, and an object thereof is to provide a reflective liquid crystal display device that can provide a bright image regardless of an operating environment without generating, for example, location-dependent uneven-brightness or moirxc3x3fringes.
To achieve the above object, a reflective liquid crystal display device of the present invention includes a liquid crystal cell in which liquid crystal is sandwiched between a pair of substrates consisting of an upper substrate and a lower substrate which are arranged so as to oppose each other, an illumination device which is provided at the upper surface side of the liquid crystal cell and which has a light source and a light guide plate, and a reflective layer provided below the liquid crystal of the liquid crystal cell. A plurality of projections or depressions are provided at the light guide plate of the illumination device so as to extend in one direction and a plurality of projections or depressions are provided at the reflective layer so as to extend in one direction. The extending direction of the projections or depressions of the light guide plate and the extending direction of the projections or depressions of the reflective layer are not parallel to each other in plan view. The xe2x80x9cprojections or depressions of the light guide platexe2x80x9d herein serve to change the reflection direction of light propagating within the light guide plate so that reflected light is emitted from a plate surface of the light guide plate to the outside thereof. The xe2x80x9cprojections or depressions of the reflective layerxe2x80x9d serve to constitute reflective surfaces that impart directivity.
The present inventors investigated the cause of generation of location-dependent uneven brightness or moirxc3x3fringes in a liquid crystal display device having a front light and a directional reflector. As a result, we found that, in the conventional configuration, since the extending direction of projections or depressions provided at a light guide plate of a front light and the extending direction of projections or depressions provided at a directional reflector are parallel to each other, light interference is generated, thus resulting in location-dependent uneven brightness or moirxc3x3fringes. Accordingly, in the liquid crystal display device of the present invention, since the extending direction of the projections or depressions of the light guide plate of the illumination device (front light) and the extending direction of the projections or depressions of the reflective layer (directional reflector) are not parallel to each other, no interference is generated. This arrangement, therefore, can prevent the generation of location-dependent uneven brightness or moirxc3xa9 fringes.
In the present invention, the above effects and advantages can be achieved by only arranging the extending direction of the projections or depressions of the light guide plate of the illumination device and the extending direction of the projections or depressions of the reflective layer such that they are not parallel to each other. In theory, as opposed to the conventional design in which the extending directions are parallel to each other, either one of the extending direction of the projections or depressions of the light guide plate and the extending direction of the projections or depressions of the reflective layer may be displaced from the other so as to be non-parallel to each other. However, since the direction in which a user commonly views the screen is usually fixed, the direction of the directivity of the reflection light is preferably fixed. In this respect, the extending direction of the projections or depressions of the light guide plate is preferably displaced from the conventional direction without changing the extending direction of the projections or depressions of the reflective layer such that the extending directions thereof are not parallel to each other.
In addition, as long as the extending direction of the projections or depressions of the light guide plate and the extending direction of the projections or depressions of the reflective layer are not parallel, that is, the angle formed by the extending directions thereof is other than 0xc2x0, it is possible to, at least, reduce the generation of location-dependent uneven brightness or moirxc3xa9 fringes, and is possible to ensure the prevention of the generation thereof if the angle is greater than 0xc2x0 by a certain degree. However, if the angle is 90xc2x0, the efficiency of utilizing light is extremely reduced, so that the display becomes significantly dark. Thus, it is desirable to set the angle to be smaller than 90xc2x0. The reason is as follows. In the design of a directional reflector, the extending direction of the projections or depressions is generally designed to be perpendicular to the direction in which light propagates within the light guide plate of the front light. Thus, when the angle formed by the extending directions thereof is set to 90xc2x0, the extending direction of the projections or depressions of the light guide plate becomes parallel to the propagation direction of light, thereby significantly reduce the efficiency of emitting light from the light guide plate.
As a specific configuration for causing light reflected from the reflective layer to have directivity, the configuration in which each of the projections or depressions of the reflective layer has an asymmetric cross-sectional shape and has a gentle-slope portion with a gentle oblique angle and a steep-slope portion with an oblique angle steeper than the gentle-slope portion may be adopted.
Such projections or depressions can be easily formed using a photolithographic technique or a transcription technique. Optimizing the oblique angle and the area of each gentle-slope portion can adjust the direction of directivity and brightness of reflected light as appropriate.
When the reflective layer is provided, it may be provided at the inner surface side of the lower substrate, or may be provided at the outer surface side of the lower substrate.
When the reflective layer is provided at the inner surface side of the lower substrate, it is possible to overcome the problem of parallax so that a sharp image can be provided. When the reflective layer is provided at the outer surface side of the lower substrate, it is possible to facilitate the manufacture of the device.
An electronic apparatus of this invention is characterized in that it includes the liquid crystal display device of the present invention.
This configuration can achieve an electronic apparatus having a liquid display portion that provides a bright image regardless of an operating environment without generating location-dependent uneven brightness or moirxc3xa9 fringes.